Electrode devices are widely used to store electrical energy, including battery cells and capacitors. For a number of reasons “super-capacitors” are gaining popularity in many energy storage applications. The reasons include availability of super-capacitors with high power densities (in both charge and discharge modes), and with energy densities approaching those of conventional rechargeable batteries.
In comparison to conventional capacitors, super-capacitors have high capacitance in relation to their volume and weight. There are two main reasons for these volumetric and weight efficiencies. First, the charge separation layers are very narrow. Their widths are typically on the order of nanometers. Second, the electrodes can be made from a porous material, having a very large effective surface area per unit volume. Because capacitance is directly proportional to the electrode area and inversely proportional to the widths of the charge separation layers, the combined effect of the large effective surface area and narrow charge separation layers a very high capacitance in comparison to conventional capacitors of similar size and weight. High capacitance of super-capacitors allows the capacitors to receive, store, and release a large amount of electrical energy.
Maximizing the charge and discharge rates is important in many applications. In backup battery applications for electronic devices (e.g., computer systems), for example, a capacitor that is used as the energy storage element during a power interruption has to be able to provide high instantaneous power, requiring high power in relation to the size of the super-capacitor.
Efficient and fast charging circuits may lead to unsafe system voltage levels. Super-capacitors act like short circuits when charging starts. If left unprotected, these charging cycles may lead to a “brown-out” where insufficient charge is being provided to the electronics device because the charge is being consumed by the super-capacitor. One way to reduce brown-outs from occurring is to slow the time required to charge the super-capacitor. Typically, a resistor is placed in series to limit the amount of current being drawn by the system. However, the resistor is subject to resistive heating which can reduce charging efficiency. In addition, the super-capacitor also becomes more resistive as the super-capacitor is charged. In a system with a fixed current output (e.g., set by the resistor described above), there is no way to speed the charging cycle.
Other solutions are available which implement a series resistor and a control circuit with an extra voltage regulator dedicated to charging the capacitor. Still other solutions implement a separate battery charger to charge the super capacitor. However, these solutions are complex and expensive to implement.